Photochromism is a property of a material to reversibly change its optical density or color, i.e. its absorption spectrum shape and/or position, under illumination. Photochromic materials are well known for use in recording and processing of optical information. Generally, an image appears on photochromes during light exposure and is stored for some time, referred to as the time of information storage, and may be erased by light, heat or other means, or fades spontaneously prior to a new recording.
Bacteriorhodopsin is a natural retinal-protein complex and was discovered to be a light-sensitive protein. Bacteriorhodopsin may be isolated from the halophilic bacteria Halobacterium salinarium, which inhabits salt lakes. The former name of this microorganism is Halobacterium halobium. Bacteriorhodopsin runs through a photochemical cycle during which the shift of the wavelength maximum of the initial absorption band takes places successively in both directions. Bacteriorhodopsin has been embedded into polymeric matrices in the form of films or blocks, Burykin et al., Optics Communications, 1985, Vol. 54, No. 2, pp. 68-71. Photochromic films have been developed containing bacteriorhodopsin or analogs thereof and polyvinyl alcohol, Birge et al., Proc. XII Ann. Internat. IEEEE-EMBS Conference, 1990, Vol. 12, No. 4/5, 1788-1789; Hampp et al., Biophys. J, 1990, Vol. 58, pp. 83-93. These films exhibit high cyclicity and high resolution owing to the bacteriorhodopsin molecule. However, the photosensitivity of these films, an important sensometric parameter, does not exceed 10.sup.-1 -10.sup.-2 J/cm.sup.2.
U.S.S.R. Author's Certificate No. 1,032,912 (1983) discloses a photochromic material containing bacteriorhodopsin analogs and a polymeric binder, such as polyvinyl alcohol or a polyvinyl-N-pyrrolidone. The disclosed material provides an information storage mime of up to several hours and a relatively wide visible spectral range of use. The energetic sensitivity of the material was disclosed as approximately 10.sup.-2 J/cm.sup.2. However, the material exhibited low energetic sensitivity and was not optically homogeneous. As a result, the signal to noise characteristics of the material were disadvantageously effected and the recorded optical information was distorted.
U.S.S.R. Author's Certificate No. 1,194,177 (1985) discloses natural retinal-protein complex in a form of an aqueous suspension of purple membranes of Halobacterium salinarium, polyvinyl alcohol as a polymeric binder and a combination of nitrogen-containing chemicals. The nitrogen-containing chemicals increased the energetic sensitivity in the visible range up to 2.times.10.sup.-3 J/cm.sup.2. However, this material exhibited insufficient optical homogeneity and insufficient energetic sensitivity in the visible spectral range.
Accordingly, a need exists for new photochromic materials exhibiting high sensitivity, high cyclicity and high energetic sensitivity in the visible spectral range.